The presence of small particles of metallic silver developed from silver halide during processing of photographic film leads to a random optical texture, which is known as film grain. The amount of film grain depends, inter alia, on the film stock.
Images or image sequences obtained with digital cameras do not contain any film grain, since there is no film and hence no grains within the film. However, a number of photographers or movie directors prefer the ‘old look’ of analog films. Therefore, artificial film grain has sometimes been used, e.g. for HD-DVD (High Density Digital Versatile Disc). Also some image processing applications for personal computers offer the possibility to add artificial film grain to digital images.
In this regard, U.S. Pat. No. 6,269,180 describes a method for compositing images derived from cinematographic film with video images or computer generated images. Color variations due to film grain are determined for film images. A similar level of variation is applied to the video images or computer generated images. In this way it is possible to add grain to non-filmed images so as to match said images to images derived from film.
Up to now artificial film grain has mainly been used for 2D images or image sequences. However, US 2009/0102914 discloses a method for storing stereoscopic 3D video on a DVD (Digital Versatile Disc) in the form of standard video data combined with enhancement data. Film grain is removed from the enhancement data and stored as metadata on the disc. During playback the film grain is re-inserted in the enhancement data. This allows for an increased coding efficiency. No specific information is given how the film grain is re-inserted in the enhancement data, and if or how film grain is treated in the standard video data.
US 2009/0160931 describes a method for image processing that supports a stereoscopic presentation. An image processing is applied in common to the two images of a stereoscopic image pair. In order to match a visual appearance of the images for a stereoscopic presentation the two images are compared. If necessary, at least one of the images is adjusted by adjusting parameters of at least one of the image capturing components providing the images. No information is given whether this processing is also applicable to film grain in the two images.
In any case, for stereoscopic 3D or multi-view images or image sequences it has been found that film grain, be it artificial or, in case of analog film, actual film grain, often leads to a rather unpleasant viewing experience. For example, when different grain is added to the different views that are presented to a viewer, the grain is unfusable by the viewer. If the same grain is added to both views, then the viewer experiences a flat blanket of grain in the convergence plane, i.e. a wall of grain. Therefore, it is generally preferred not to add any film grain to stereoscopic or multi-view images and to remove any actual film grain by filtering. However, in an interview in the Web Portal HD HEAD it has been proposed to convert the grain in such way that it has the same depth as the object it occludes. The visual effect is that the surfaces are grainy (http://www.hdhead.com/?p=279). Unfortunately, no information is given how this can be achieved. Also, due to incomplete depth information this solution is likely to generate a rather large amount of grain artifacts.
WO 2011/142734 discloses a method for applying film grain to stereoscopic images. First film grain is added to a first view in dependence of depth values derived from a depth map. The film grain is then warped and added to the second view. Finally, holes are filled with further film grain based on the depth values for the second view.